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//! Render a docker image.

// Docker image format is specified at
// https://github.com/moby/moby/blob/v17.05.0-ce/image/spec/v1.md

use errors::*;
use libflate::gzip;
use std::{fs, path};
use tar;

/// Unpack an ordered list of layers to a target directory.
///
/// Layers must be provided as gzip-compressed tar archives, with lower layers
/// coming first. Target directory must be an existing absolute path.
pub fn unpack(layers: &[Vec<u8>], target_dir: &path::Path) -> Result<()> {
    if !target_dir.is_absolute() || !target_dir.exists() || !target_dir.is_dir() {
        bail!("wrong target path");
    }
    for l in layers {
        // Unpack layers
        let gz_dec = gzip::Decoder::new(l.as_slice())?;
        let mut archive = tar::Archive::new(gz_dec);
        archive.set_preserve_permissions(true);
        archive.set_unpack_xattrs(true);
        archive.unpack(target_dir)?;

        // Clean whiteouts
        let gz_dec = gzip::Decoder::new(l.as_slice())?;
        let mut archive = tar::Archive::new(gz_dec);
        for entry in archive.entries()? {
            let file = entry?;
            let path = file.path()?;
            let parent = path.parent().unwrap_or(path::Path::new("/"));
            if let Some(fname) = path.file_name() {
                let wh_name = fname.to_string_lossy();
                if wh_name.starts_with(".wh.") {
                    let rel_parent = path::PathBuf::from("./".to_string() + &parent.to_string_lossy());

                    // Remove real file behind whiteout
                    let real_name = wh_name.trim_left_matches(".wh.");
                    let abs_real_path = target_dir.join(&rel_parent).join(real_name);
                    fs::remove_dir_all(abs_real_path)?;

                    // Remove whiteout place-holder
                    let abs_wh_path = target_dir.join(&rel_parent).join(fname);
                    fs::remove_dir_all(abs_wh_path)?;
                };
            }
        };
    };
    Ok(())
}